1. Field of the Invention
The present invention relates to a frequency error detecting apparatus suitable for use in a receiving apparatus for receiving a digitally-modulated signal such as an FSK (Frequency Shift Keying) signal, which is adapted to detect an error, i.e., a difference between the center frequency of a received signal and the center frequency which has been generated by the receiving apparatus.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional frequency error detecting apparatus. In the same drawing, there are shown an input terminal 1 to which a signal to be received is applied, a counting unit 2 for counting the number of cycles of the received signal within a specific time period, an observation timing generating apparatus 3 for generating a timing cycle for the error detection, a comparing unit 4 for making comparison between a value counted by the counting unit 2 and a predetermined value, and an output terminal 10.
An operation of the frequency error detecting apparatus will now be described. A description will be made herein in a case where a binary frequency-shift keying (hereinafter called "binary FSK") signal is inputted as a signal to be received. First of all, the counting unit 2 counts the number of cycles of a signal inputted to the input terminal 1 over a predetermined time period. For example, when a signal to be inputted to counting unit 2 is a single tone signal of 1,750 Hz, a value to be counted by the counting unit 2 after 1 second has elapsed is 1,750. The observation timing generating apparatus 3 applies an observation start timing signal and an observation completion timing signal to the counting unit 2. When the counting unit 2 responds to the observation start timing signal from the observation timing generating apparatus 3, it starts counting, while when it responds to the observation completion timing signal therefrom, it outputs a counted value.
The binary FSK signal is used to represent two data symbols, "0" and "1". When the center frequency of the signal is set to f.sub.c, f.sub.c +.DELTA.f is assigned to the data "0" whereas f.sub.c -.DELTA.f is assigned to the data "1". .DELTA.f will hereinafter be called a displacement or shift frequency (.DELTA.f is a positive value). When data produced on the transmission side are added up over a sufficiently long period of time, the added-up data "0"/"1" ratios are considered as being equal to each other. In other words, a value outputted from the counting unit 2 should be equal to (the center frequency f.sub.c).times.(observed period). Then, the comparing unit 4 compares the counted value and nf.sub.c (R)T, where T is the inverse of the data transmission rate (for example, 1/300 in the case of a 300 bps rate), n is the number of bits of data received for the observed period, and nT represents the observed period. In addition, f.sub.c (R) represents the center frequency which has been set in the receiving apparatus. Thus, a value (f.sub.c -f.sub.c (R)).times.(observed period) is obtained as a result of the above comparison by the comparing unit 4. If the resultant value is divided by the observed period, a frequency error can be obtained.
Incidentally, the comparing unit 4 can obtain a frequency error by merely comparing a value inputted from the counting unit 2 and f.sub.c (R) provided that the counting unit 2 outputs a value equal to the counted value divided by the observed period. Then, the resultant frequency error is outputted from the output terminal 10.
Since the conventional frequency error detecting apparatus is constructed as described above, it is necessary to make the observed period relatively long for purposes of an improvement in the accuracy of error detection. In addition, since the probabilities of occurrence of the data "0" and "1" are assumed as being equal to each other, errors in detecting become greater where data generated on the transmission side is shifted as to its distribution, thereby causing a problem.